Stainless steel sheet and method of manufacturing same, edged tool, and cutlery

ABSTRACT

Provided is a stainless steel sheet having a predetermined chemical composition, in which a total volume fraction of Cr-based carbides with a grain size of 2.0 μm or more is 10% or less.

TECHNICAL FIELD

This disclosure relates to a stainless steel sheet having high hardness and good surface quality suitable for use in edged tools such as kitchen knives, scissors, and medical scalpels, cutlery such as table knives, forks and spoons, and precision tools such as tweezers.

BACKGROUND

Stainless steel sheets may be used as materials for edged tools such as kitchen knives, scissors, and medical scalpels, and precision tools such as tweezers.

For example, in the case of kitchen knives, a stainless steel sheet is blanked or forged into a predetermined shape by, for example, press working. Then, the stainless steel sheet, which has been worked into the predetermined shape, is hardened by quenching or quenching and tempering treatments. The hardened stainless steel sheet is then subjected to polishing for edging (a process in which a portion to be a cutting edge is thinned by polishing) and other processes to make the final product (kitchen knife).

The stainless steel used for the above edged tool and precision tool applications includes, for example, 13 mass % Cr-0.3 mass % C steel (SUS420J2 specified in JIS G 4304 and JIS G 4305).

Edged tools and precision tools are also required to reduce the frequency of maintenance such as sharpening by minimizing the deterioration of sharpness caused by wear of the cutting edge and rusting.

In recent years, this demand has been particularly high, and there is a growing market need for high-quality edged tools with high hardness that can ensure sufficient corrosion resistance while having high sharpness and suppressing the decline in sharpness due to wear of the cutting edge for the long term.

The stainless steel used for such high-quality edged tools with high hardness includes, for example, 14 mass % Cr-0.5 mass % C steel according to to European unified standard: EN 1.4116. The 14 mass % Cr-0.5 mass % C steel according to European unified standard: EN 1.4116 is steel with increased hardness compared to 13 mass % Cr-0.3 mass % C.

JP5010819B (PTL 1) describes:

“a stainless steel strip comprising C: 0.88 mass % or more and 1.2 mass % or less, Cr: 12.5 mass % or more and 16.50 mass % or less, Si: 0.05 mass % or more and 0.20 mass % or less, N: 0.001 mass % or more and 0.02 mass % or less, Mn: 1.0 mass % or less, Cu: 1.0 mass % or less, P: 0.03 mass % or less, S: 0.010 mass % less, and Ni: 1.0 mass % or less, with the balance being Fe and inevitable impurities”.

CITATION LIST Patent Literature

-   PTL 1: JP5010819B

SUMMARY Technical Problem

However, when the 14 mass % Cr-0.5 mass % C steel according to the European unified standard: 1.4116 and a steel sheet obtained from the stainless steel strip disclosed in PTL 1 are polished or edged, a stripe pattern may occur along the rolling direction, which greatly affects the appearance.

When such stripe pattern occurs, it is necessary to remove it by adding a polishing process or by other means. However, the additional polishing process increases manufacturing costs. Further, if the stripe pattern is significant, the stripe pattern may not be removed completely, or the amount of polishing required to remove the stripe pattern may be too large to obtain the desired shape. This results in a significant reduction in yield rate and productivity.

Therefore, there is a need to develop stainless steel sheets that exhibit high hardness when used as products and good surface quality with reduced generation of stripe patterns when worked into products.

It could thus be helpful to provide a stainless steel sheet that exhibit high hardness when used as products (hereinafter simply referred to as high hardness) and good surface quality with reduced generation of stripe patterns when worked into a product (hereinafter simply referred to as good surface quality).

It could also be helpful to provide a method of manufacturing the stainless steel sheet.

Furthermore, it could be helpful to provide edged tools and cutlery made of the stainless steel sheet.

As mentioned above, the disclosed stainless steel sheet refers to a steel sheet that provides high hardness when used as products such as edged tools and cutlery. In detail, the disclosed stainless steel sheet includes not only a steel sheet after hardening (after quenching treatment) but also a steel sheet that is used as a product material before hardening (before quenching treatment).

Solution to Problem

We therefore made intensive studies to achieve the objects stated above.

We first investigated the cause of the stripe pattern that occurs when polishing or edging (hereinafter simply referred to as polishing) is applied to 14 mass % Cr-0.5 mass % C steel according to the European unified standard: EN 1.4116.

Specifically,

-   -   a steel sheet having a chemical composition of 14 mass % Cr-0.5         mass % C steel according to the European unified standard:         EN1.4116 (hereinafter simply referred to as Steel Sheet a), and     -   a steel sheet having a chemical composition of 13 mass % Cr-0.3         mass % C steel corresponding to SUS420J2 specified in JIS G 4304         and JIS G 4305 (hereinafter simply referred to as Steel Sheet b)         were each manufactured under the same conditions by a         conventionally known method, and the manufactured steel sheets         were polished under the same conditions.

As a result, no stripe pattern occurred on Steel Sheet b even after polishing. On the other hand, on Steel Sheet a, stripe patterns occurred when polishing was applied.

The above results led us to believe the following.

In detail, due to the difference in chemical composition, the precipitation state of precipitates is significantly different between Steel Sheets a and b even when the steel sheets are manufactured under the same manufacturing conditions. The difference in the precipitation state of precipitates generates the stripe pattern on Steel Sheet a.

Based on this idea, we observed the metallic structure of Steel Sheets a and b and contrasted them in detail.

As a result, we found that, on Steel Sheet a where the stripe pattern is generated, coarse Cr carbides are continuously present in the metallic structure in the rolling direction as illustrated in FIG. 2 , and that this is the cause of the stripe pattern.

In detail, Cr-based carbides are harder than the base metal of the stainless steel sheet (both before and after quenching). Therefore, when coarse Cr-based carbides are present in the metallic structure, the amount of polishing is lower in the area where such Cr-based carbides are present than in other areas. As a result, after polishing, local convex portions are generated and become apparent as stripe patterns.

In particular, in the chemical composition of Steel Sheet a (14 mass % Cr—0.5 mass % C steel according to the European unified standard: EN 1.4116), in order to obtain higher hardness, a larger amount of C and Cr are contained than in Steel Sheet b (13 mass % Cr—0.3 mass % C steel). Therefore, although a large amount of coarse Cr carbides are not formed on Steel Sheet b even when it is manufactured by a conventionally known method, a large amount of coarse Cr carbides are formed on Steel Sheet a manufactured under the same conditions, resulting in the formation of stripe patterns.

Based on the above, we further investigated and made the following findings.

In detail, Cr-based carbides with a grain size of 2.0 μm or more deeply affect the generation of stripe patterns during polishing. The generation of stripe patterns during polishing is greatly suppressed by suppressing the formation of such coarse Cr-based carbides as much as possible, especially by suppressing the volume fraction of Cr-based carbides with a grain size of 2.0 μm or more to 10% or less.

We studied further and made the following findings.

In detail, the coarse Cr carbides described above are formed along the casting direction near the boundaries between columnar crystals and equiaxial crystals in the slab section during casting. In addition, under the conventionally known general manufacturing conditions, the coarse Cr carbonitrides formed during casting still remain in the rolling direction (the same direction as the casting direction) even after hot rolling, hot-rolled sheet annealing, cold rolling, and cold-rolled sheet annealing processes after the casting process.

Based on the above findings, we investigated methods to prevent the formation of coarse Cr-based carbides while obtaining high hardness.

As a result, we found that the following is important.

(1) The chemical composition should be appropriately adjusted, in particular, the C content should be adjusted to 0.45 mass % or more and 0.60 mass % or less and the Cr content should be adjusted to 13.0 mass % or more and less than 16.0 mass %. (2) Then, the heating, hot rolling and hot-rolled sheet annealing conditions of the steel slab should be properly controlled.

Specifically,

-   -   (a) The steel slab is held at a temperature of 1200° C. or         higher and 1350° C. or lower for at least 30 minutes.     -   (b) In the hot rolling, at least three rolling passes with a         finish temperature of 1050° C. or higher and a rolling reduction         of 20% or more are performed.     -   (c) Further, the coiling temperature of the hot-rolled steel         sheet is set to 600° C. or higher. The formation of coarse         Cr-based carbides can be thus suppressed and the generation of         stripe patterns can be prevented during polishing, even when the         C and Cr contents exceed a certain amount.

We believe that the reason why the formation of coarse Cr-based carbides is suppressed by controlling the manufacturing conditions as described above is as follows.

In detail, as described in (2)(a) above, holding the steel slab at a temperature of 1200° C. or higher and 1350° C. or lower for at least 30 minutes promotes the dissolution of the coarse Cr-based carbides formed in the casting process into the austenite phase (the Cr-based carbides are decomposed into Cr atoms, C atoms, and so on, and incorporated into the austenite phase in atomic form).

In this state, by performing rolling passes in hot rolling at high temperature and high rolling reduction as in (2)(b) above, the dissolution of the Cr-based carbides into the austenite phase is further promoted. In addition, rolling strain is effectively applied to the center portion in steel slab thickness direction. This eliminates the coarse Cr-based carbides that have formed along the casting direction near boundaries between columnar crystals and equiaxial crystals in the steel slab. Further, it promotes the diffusion of elements at dislocations (atomic migration through dislocations, which are lattice defects). This further promotes the dissolution of Cr-based carbides into the austenite phase. Furthermore, the crystal grains of the austenite phase are refined by promoting dynamic and/or static recrystallization of the austenite phase. This increases the precipitation site of Cr-based carbides that precipitate from the grain boundaries of the austenite phase during the coiling of the hot-rolled steel sheet in (2)(c) above to refine re-precipitating Cr-based carbides. Recrystallization is a phenomenon in which crystal grains containing little strain are formed from within strained crystal grains or from strained crystal grain boundaries.

Through the synergistic effects described above, the formation of coarse Cr-based carbides can be suppressed and the generation of stripe patterns can be prevented during polishing, even when certain amounts of C and Cr are contained.

The present disclosure is based on these findings and further studies.

We thus provide the following.

1. A stainless steel sheet comprising a chemical composition containing (consisting of), in mass %,

C: 0.45% or more and 0.60% or less,

Si: 0.05% or more and 1.00% or less,

Mn: 0.05% or more and 1.00% or less,

P: 0.05% or less,

S: 0.020% or less,

Cr: 13.0% or more and less than 16.0%,

Ni: 0.10% or more and 1.00% or less, and

N: 0.010% or more and 0.200% or less

with the balance being Fe and inevitable impurities,

wherein a total volume fraction of Cr-based carbides with a grain size of 2.0 μm or more is 10% or less.

2. The stainless steel sheet according to 1, wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of

Mo: 0.05% or more and 1.00% or less,

Cu: 0.05% or more and 1.00% or less, and

Co: 0.05% or more and 0.50% or less.

3. The stainless steel sheet according to 1. or 2, wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of

Al: 0.001% or more and 0.100% or less,

Ti: 0.01% or more and 0.10% or less

Nb: 0.01% or more and 0.10% or less,

V: 0.05% more and 0.50% or less,

Zr: 0.01% or more and 0.10% or less,

Mg: 0.0002% or more and 0.0050% or less,

B: 0.0002% or more and 0.0050% or less,

Ca: 0.0003% or more and 0.0030% or less, and

REM: 0.01% or more and 0.10% or less.

4. A method of manufacturing the stainless steel according to any of 1. to 3, the method comprising:

a first step in which a steel slab having the chemical composition according to any of 1. to 3. is held at 1200° C. or higher and 1350° C. or lower for at least 30 minutes,

a second step in which the steel slab is hot rolled to obtain a hot-rolled steel sheet and the hot-rolled steel sheet is coiled, and

a third step in which the hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled and annealed steel sheet

wherein in hot rolling of the second step, at least three rolling passes with a finish temperature of 1050° C. or higher and a rolling reduction of 20% or more are performed, and a coiling temperature of the hot-rolled steel sheet is 600° C. or higher,

and in the hot-rolled sheet annealing of the third step, a holding temperature is 750° C. or higher and 900° C. or lower and a holding time is 10 minutes or more.

5. The method of manufacturing the stainless steel sheet according to 4, comprising the fourth step in which the hot-rolled and annealed steel sheet is cold rolled to obtain a cold-rolled steel sheet. 6. The method of manufacturing the stainless steel sheet according to 5, comprising the fifth step in which the cold-rolled steel sheet is subjected to cold-rolled sheet annealing to obtain a cold-rolled and annealed steel sheet

wherein in the cold-rolled sheet annealing, a holding temperature is 700° C. or higher and 850° C. or lower and a holding time is 5 seconds or more.

7. The method of manufacturing the stainless steel sheet according to any of 4. to 6, comprising the sixth step in which the hot-rolled and annealed steel sheet, cold-rolled steel sheet, or cold-rolled and annealed steel sheet is subjected to quenching treatment

wherein in the quenching treatment, a holding temperature is 950° C. or higher and 1200° C. or lower, a holding time is 5 seconds or more and 30 minutes or less, and an average cooling rate after holding is 1° C./s or more.

8. The method of manufacturing the stainless steel sheet according to 7, comprising the seventh step in which the steel sheet subjected to quenching treatment is subjected to a tempering treatment

wherein in the tempering treatment, a holding temperature is 100° C. or higher and 800° C. or lower and a holding time is 5 minutes or more.

9. An edged tool made of the stainless steel sheet according to any of 1. to 3. 10. Cutlery made of the stainless steel sheet according to any of 1. to 3.

Advantageous Effect

According to the present disclosure, a stainless steel sheet with high hardness and good surface quality can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is an optical microscope micrograph of Example No. 1;

FIG. 2 is an optical microscope micrograph of Comparative Example No. 30; and

FIG. 3 schematically illustrates the condition of a test piece applied with cutting work in the evaluation of surface quality.

DETAILED DESCRIPTION

The presently disclosed techniques will be described below by way of embodiments.

First, the chemical composition of the stainless steel sheet according to one of the disclosed embodiments will be described. The % representations below indicating the chemical composition are in mass % unless stated otherwise.

C: 0.45% or More and 0.60% or Less

C has the effect of hardening the martensite phase obtained by the quenching treatment. If the C content is less than 0.45%, the hardness after quenching treatment is insufficient and the sharpness required for high-grade edged tools cannot be obtained adequately. On the other hand, when the C content exceeds 0.60%, the generation of coarse carbides cannot be sufficiently suppressed and good surface quality cannot be obtained, even when the manufacturing conditions are properly controlled. In addition, quench cracks are more likely to occur during the quenching treatment, making it difficult to manufacture edged tools in a stable manner.

The C content is therefore set to 0.45% or more and 0.60% or less. The C content is preferably 0.55% or less, and more preferably 0.50% or less.

Si: 0.05% or More and 1.00% or Less

Si acts as a deoxidizer in steelmaking. To achieve this effect, the Si content is set to 0.05% or more. However, when the Si content exceeds 1.00%, the steel sheet becomes excessively hardened before quenching treatment and does not have sufficient workability when formed into a specified shape such as an edged tool.

The Si content is therefore set to 0.05% or more and 1.00% or less. The Si content is preferably 0.20% or more. The Si content is preferably 0.60% or less.

Mn: 0.05% or More and 1.00% or Less Mn promotes the formation of austenite phase and improves hardenability. To achieve this effect, the Mn content is set to 0.05% or more. However, when the Mn content exceeds 1.00%, corrosion resistance is reduced.

The Mn content is therefore set to 0.05% or more and 1.00% or less. The Mn content is preferably 0.40% or more. The Mn content is preferably 0.80% or less.

P: 0.05% or Less

P is an element that contributes to intergranular fracture due to grain boundary segregation. Therefore, it is desirable to reduce P as much as possible.

The P content is therefore set to 0.05% or less. The P content is preferably 0.04% or less, and more preferably 0.03% or less.

No lower limit is placed on the P content. However, since excessive dephosphorization leads to cost increase, a P content of 0.005% or more is preferred.

S: 0.020% or Less

S is an element that exists in steel as sulfide inclusions such as MnS, which reduces ductility, corrosion resistance, and other properties. Therefore, it is desirable to reduce S as much as possible.

The S content is therefore set to 0.020% or less. The S content is preferably 0.015% or less.

No lower limit is placed on the S content. However, since excessive desulfurization leads to cost increase, a S content of 0.0005% or more is preferred.

Cr: 13.0% or More and Less than 16.0%

Cr has the effect of improving corrosion resistance. To achieve this effect, the Cr content is set to 13.0% or more. However, when the Cr content is 16.0% or more, the amount of austenite formed during heating and holding in the quenching treatment decreases. As a result, the martensite phase obtained after the quenching treatment is reduced and sufficient hardness cannot be obtained. Therefore, the Cr content is set to 13.0% or more and less than 16.0%. The Cr content is preferably 14.0% or more. The Cr content is preferably 15.5% or less, and more preferably 15.0% or less.

Ni: 0.10% or More and 1.00% or Less

Ni improves corrosion resistance and toughness after quenching. To achieve this effect, the Ni content is set to 0.10% or more. However, when the Ni content exceeds 1.00%, the effect is saturated. The increase in the amount of solute Ni also causes the steel sheet to become excessively hard before the quenching treatment, making it difficult to obtain sufficient workability when forming the steel sheet into a specified shape such as an edged tool.

The Ni content is therefore set to 0.10% or more and 1.00% or less. The Ni content is preferably 0.15% or more, and more preferably 0.20% or more. The Ni content is preferably 0.80% or less, and more preferably 0.60% or less.

N: 0.010% or More and 0.200% or Less

As with C, N has the effect of hardening the martensite phase obtained by the quenching treatment. N also improves corrosion resistance after the quenching treatment. To achieve this effect, the N content is set to 0.010% or more. However, when the N content exceeds 0.200%, blow holes are generated during casting, which induces the generation of surface defects.

The N content is therefore set to 0.010% or more and 0.200% or less. The N content is preferably 0.015% or more, and more preferably 0.020% or more. The N content is preferably 0.150% or less, and more preferably 0.100% or less.

The basic chemical composition of the stainless steel sheet according to one of the disclosed embodiments has been described.

The chemical composition further contains at least one selected from the group consisting of Mo: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less and Co: 0.05% or more and 0.50% or less,

and/or

at least one selected from the group consisting of Al: 0.001% or more and 0.100% or less, Ti: 0.01% or more and 0.10% or less, Nb: 0.01% or more and 0.10% or less, V: 0.05% or more and 0.50% or less, Zr: 0.01% or more and 0.10% or less, Mg: 0.0002% or more and 0.0050% or less, B: 0.0002% or more and 0.0050% or less, Ca: 0.0003% or more and 0.0030% or less, and REM: 0.01% or more and 0.10% or less.

Mo: 0.05% or More and 1.00% or Less

Mo has the effect of improving corrosion resistance. To achieve this effect, the Mo content is preferably 0.05% or more. However, when the Mo content exceeds 1.00%, the amount of austenite formed during heating and holding in the quenching treatment decreases, and sufficient hardness cannot be obtained after the quenching treatment.

Therefore, when Mo is contained, the Mo content is preferably 0.05% or more and 1.00% or less. The Mo content is more preferably 0.10% or more, and further preferably 0.50% or more. The Mg content is more preferably 0.80% or less, and further preferably 0.65% or less.

Cu: 0.05% or More and 1.00% or Less

Cu has the effect of improving temper softening resistance in the steel sheet after the quenching treatment. To achieve this effect, the Cu content is preferably 0.05% or more. However, when the Cu content exceeds 1.00% corrosion resistance is reduced.

Therefore, when Cu is contained, the Cu content is preferably 0.05% or more and 1.00% or less. The Cu content is more preferably 0.10% or more. The Cu content is more preferably 0.50% or less, and further preferably 0.20% or less.

Co: 0.05% or More and 0.50% or Less

Co has the effect of improving toughness. To achieve this effect, the Co content is preferably 0.05% or more. However, when the Co content exceeds 0.50%, the steel sheet does not have sufficient workability when formed into a predetermined shape such as an edged tool, prior to the quenching treatment.

Therefore, when Co is contained, the Co content is preferably 0.05% or more and 0.50% or less. The Co content is more preferably 0.10% or more. The Co content is more preferably 0.20% or less.

Al: 0.001% or More and 0.100% or Less

Al, as with Si, acts as a deoxidizer. To achieve this effect, the Al content is preferably 0.001% or more. However, when the Al content exceeds 0.100%, the hardenability is reduced.

Therefore, when Al is contained, the Al content is preferably 0.001% or more and 0.100% or less. The Al content is more preferably 0.050% or less, and further preferably 0.010% or less.

Ti: 0.01% or More and 0.10% or Less

As with Cr, Ti is an element that has a high affinity for C and N and forms carbides in steel. Ti also has the effect of improving temper softening resistance. This makes it possible to improve toughness while suppressing softening when tempering is performed. To achieve this effect, the Ti content is preferably 0.01% or more. However, when the Ti content exceeds 0.10% the effect is saturated and instead, toughness is reduced.

Therefore, when Ti is contained, the Ti content is preferably 0.01% or more and 0.10% or less. The Ti content is more preferably 0.02% or more. The Ti content is more preferably 0.05% or less.

Nb: 0.01% or More and 0.10% or Less

As with Ti, Nb is an element that has a high affinity for C and N and forms carbides in steel. Nb also has the effect of improving temper softening resistance. This makes it possible to improve toughness while suppressing softening when tempering is performed. To achieve this effect, the Nb content is preferably 0.01% or more. However, when the Nb content exceeds 0.10%, the effect is saturated. In addition, a decrease in toughness may occur due to precipitation of intermetallic compounds.

Therefore, when Nb is contained, the Nb content is preferably 0.01% or more and 0.10% or less. The Nb content is more preferably 0.02% or more. The Nb content is more preferably 0.05% or less.

V: 0.05% More and 0.50% or Less

As with Ti and Nb, V is an element that has a high affinity for C and N and forms carbides in steel. V also has the effect of improving temper softening resistance. This makes it possible to improve toughness while suppressing softening when tempering is performed. To achieve this effect, the V content is preferably 0.05% or more. However, when the V content exceeds 0.50%, the effect is saturated. In addition, a decrease in toughness may occur due to precipitation of intermetallic compounds.

Therefore, when V is contained, the V content is preferably 0.05% or more and 0.50% or less. The V content is more preferably 0.10% or more. The V content is more preferably 0.30% or less, and further preferably 0.20% or less.

Zr: 0.01% or More and 0.10% or Less

As with Ti and Nb, Zr is an element that has a high affinity for C and N, and forms carbides in steel. Zr also has the effect of improving temper softening resistance. This makes it possible to improve toughness while suppressing softening when tempering is performed. To achieve this effect, the Zr content is preferably 0.01% or more. However, when the Zr content exceeds 0.10%, the effect is saturated. In addition, a decrease in toughness may occur due to precipitation of intermetallic compounds.

Therefore, when Zr is contained, the Zr content is preferably 0.01% or more and 0.10% or less. The Zr content is more preferably 0.02% or more. Further, the Zr content is more preferably 0.05% or less.

Mg: 0.0002% or More and 0.0050% or Less

Mg has the effect of increasing the equiaxial crystal rate of the slab and improving workability and toughness. To achieve this effect, the Mg content is preferably 0.0002% or more. However, when the Mg content exceeds 0.0050%, the surface characteristics of the steel sheet may deteriorate.

Therefore, when Mg is contained, the Mg content is preferably 0.0002% or more and 0.0050% or less. The Mg content is more preferably 0.0010% or more. Further, the Mg content is more preferably 0.0020% or less.

B: 0.0002% or More and 0.0050% or Less

B has the effect of improving hot workability during casting and hot rolling. B also segregates at the grain boundaries of ferrite phase and austenite phase to increase grain boundary strength. This suppresses cracking during casting and hot rolling. To achieve this effect, the B content is preferably 0.0002% or more. However, when the B content exceeds 0.0050%, the steel sheet does not have sufficient workability when formed into a predetermined shape such as an edged tool, prior to the quenching treatment. It also leads to a decrease in toughness.

Therefore, when B is contained, the B content is preferably 0.0002% or more and 0.0050% or less. The B content is more preferably 0.0005% or more. The B content is more preferably 0.0030% or less, and further preferably 0.0020% or less.

Ca: 0.0003% or More and 0.0030% or Less

Ca has the effect of refining inclusions formed during smelting and continuous casting and is particularly effective in preventing nozzle blockage in continuous casting. To achieve this effect, the Ca content is preferably 0.0003% or more. However, when the Ca content exceeds 0.0030% corrosion resistance may be reduced due to the formation of CaS.

Therefore, when Ca is contained, the Ca content is preferably 0.0003% or more and 0.0030% or less. The Ca content is more preferably 0.0005% or more, and further preferably 0.0007% or more. The Ca content is more preferably 0.0020% or less, and further preferably 0.0015% or less.

REM: 0.01% or More and 0.10% or Less

Rare Earth Metals (REM) have the effect of improving hot ductility. REM also has the effect of suppressing crack and rough skin on the edges of the steel sheet during hot rolling. To achieve this effect, the REM content is preferably 0.01% or more. However, when the REM content exceeds 0.10% the effect is saturated. Further, REM is an expensive element.

Therefore, when REM is contained, the REM content is preferably 0.01% or more and 0.10% or less. The REM content is more preferably 0.05% or less.

The balance other than the aforementioned components are Fe and inevitable impurities.

Next, the metallic structure of the stainless steel sheet according to one of the disclosed embodiments will be described.

The metallic structure of the stainless steel sheet according to one of the disclosed embodiments changes its main structure before and after quenching treatment.

For example, when processing the stainless steel sheet according to one of the disclosed embodiments into a product, the steel sheet is first blanked or forged into a predetermined shape by, for example, press working at a stage where the steel sheet is not hardened. Then, the steel sheet, which has been worked into the predetermined shape, is hardened by quenching treatment or quenching and tempering treatments. That is, before and after the quenching treatment, the main structure is changed, specifically from ferrite phase to martensite phase.

However, Cr-based carbides with a grain size of 2.0 μm or more do not change much and are mostly maintained before and after the quenching treatment.

Therefore, in the metallic structure of the stainless steel sheet according to one of the disclosed embodiments, whether before or after quenching treatment, it is extremely important that the volume fraction of Cr carbides of a grain size: 2.0 μm or less be 10% or less.

Volume Fraction of Cr Carbides with Grain Size of 2.0 μm or More: 10% or Less

Cr-based carbides are harder than the base metal of the stainless steel sheet (both before and after quenching). Therefore, when polishing or edging, etc. is performed with a large amount of Cr-based carbides, especially Cr-based carbides with a grain size of 2.0 μm or more, present in the metallic structure, the amount of polishing will be lower in the area where such Cr-based carbides are present than in other areas. As a result, after polishing, local convex portions are generated and become apparent as stripe patterns.

Therefore, the volume fraction of Cr-based carbides with a grain size of 2.0 μm or more should be 10% or less. The volume fraction of Cr-based carbides with a grain size of 2.0 μm or more is preferably less than 5%, and more preferably 2% or less. The volume fraction of Cr-based carbides with a grain size of 2.0 μm or more may be 0%.

For Cr-based carbides with a grain size of less than 2.0 μm, they do not produce enough irregularities to be discernible to the naked eye during polishing and are not involved in the generation of the stripe pattern. Therefore, the volume fraction of Cr-based carbides with a grain size of less than 2.0 μm is not limited.

The Cr-based carbides here are mainly Cr₂₃C₆. In addition, Cr carbides in which a part of Cr is replaced by elements such as Fe, Mn, Ti, Nb, V, Zr, and a part of C is replaced by N are also referred to Cr carbide.

In addition, the microstructure other than the Cr-based carbides in the stainless steel sheet according to one of the disclosed embodiments is a metallic structure that has a total volume fraction of ferrite and martensite phases of 95% or more, more preferably 98% or more. The total volume fraction of ferrite and martensite phases may be 100%. The residual microstructure other than the ferrite phase, martensite phase and Cr-based carbides mentioned above includes a retained austenite phase, other precipitates (Cr carbides with a grain size of less than 2.0 μm), and inclusions (e.g., oxides such as Al and Si and sulfides such as Mn). The volume fraction of residual microstructure is preferably 5% or less and more preferably 2% or less. The volume fraction of residual microstructure may be 0%.

The stainless steel sheet according to one of the disclosed embodiments includes, for example, steel sheets both before and after quenching, such as a hot-rolled steel sheet, a hot-rolled and annealed steel sheet, a cold-rolled steel sheet, and a cold-rolled and annealed steel sheet, and steel sheets obtained by subjecting the abovementioned steel sheets to quenching and/or tempering treatments (the quenched steel sheet and tempered steel sheets as described below).

In the stages of hot-rolled steel sheet, hot-rolled and annealed steel sheet, cold-rolled steel sheet, and cold-rolled and annealed steel sheet, the microstructure other than the Cr-based carbides is mainly ferrite phase.

Specifically, it is a metallic structure in which the volume fraction of ferrite phase is 80% or more, preferably 90% or more, more preferably 95% or more, and still more preferably 98% or more. The volume fraction of ferrite phase may be 100%. The residual microstructure other than the ferrite phase and Cr-based carbides mentioned above includes a martensite phase, a retained austenite phase, other precipitates (Cr carbides with a grain size of less than 2.0 μm), and inclusions (e.g., oxides such as Al and Si and sulfides such as Mn). The volume fraction of residual microstructure is preferably 20% or less, more preferably 10% or less, further preferably 5% or less, and still further preferably 2% or less. The volume fraction of residual microstructure may be 0%.

The hot-rolled steel sheet includes not only a hot-rolled steel sheet but also a steel sheet obtained by applying pickling or other oxide scale removal treatment to the hot-rolled steel sheet. Further, the hot-rolled and annealed steel sheet includes, in addition to a steel sheet obtained by applying hot-rolled sheet annealing to the hot-rolled steel sheet, a steel sheet obtained by further applying pickling or other oxide scale removal treatments to the steel sheet obtained by applying hot-rolled sheet annealing. The cold-rolled steel sheet includes not only a cold-rolled steel sheet but also a steel sheet obtained by applying pickling or other oxide scale removal treatments to the cold-rolled steel sheet.

Furthermore, in steel sheets obtained by applying quenching treatment to the hot-rolled steel sheet, hot-rolled and annealed steel sheet, cold-rolled steel sheet, and cold-rolled and annealed steel sheet (hereinafter also referred to as quenched steel sheet), the structure other than the Cr-based carbides is mainly martensite phase.

Specifically, it is a metallic structure in which the volume fraction of martensite phase is 80% or more, preferably 90% or more, more preferably 95% or more, and still more preferably 98% or more. The volume fraction of martensite phase may be 100%. The residual microstructure other than the martensite phase and Cr-based carbides mentioned above includes a ferrite phase, a retained austenite phase, other precipitates (Cr carbides with a grain size of less than 2.0 μm), and inclusions (e.g., oxides such as Al and Si and sulfides such as Mn). The volume fraction of residual microstructure is preferably 20% or less, more preferably 10% or less, further preferably 5% or less, and still further preferably 2% or less. The volume fraction of residual microstructure may be 0%.

Since the quenching treatment hardens a steel sheet, the Rockwell hardness of the quenched steel sheets is HRC55 or more.

In addition, in steel sheets obtained by applying a tempering treatment to the quenched steel sheets (hereinafter referred to as “tempered steel sheet”), the microstructure other than Cr-based carbides is mainly martensitic phase (sometimes referred to as “tempered martensite phase”) in which dislocation density and solute C and N are reduced compared to those after the quenching treatment, and the volume fraction of martensite before the tempering treatment is almost maintained.

Specifically, it is a metallic structure in which the volume fraction of martensite phase is 80% or more, preferably 90% or more, more preferably 95% or more, and still more preferably 98% or more. Further, in the metallic structure, the volume fraction of ferrite phase is 20% or less, preferably 10% or less, more preferably 5% or less, and still more preferably 2% or less. The residual microstructure other than the ferrite phase, martensite phase and Cr-based carbides mentioned above includes a retained austenite phase, other precipitates (Cr carbides with a grain size of less than 2.0 μm), and inclusions (e.g., oxides such as Al and Si and sulfides such as Mn). The volume fraction of residual microstructure is preferably 5% or less and more preferably 2% or less.

Here, tempering is performed to adjust the hardness and durability of a steel sheet that have been hardened by quenching treatment, and the hardness is reduced in tempered steel sheet compared to quenched steel sheet before tempering. Specifically, the tempered steel sheet has a Rockwell hardness of HRC 40-50.

The volume fraction of Cr-based carbides with a grain size of 2.0 μm or more is measured as described below.

In detail, a test piece for microstructure observation is taken from the center portion of the sheet width of a steel sheet to be used as a sample. The cross section of the test piece in the rolling direction is then mirror polished, etched using picric acid-hydrochloric acid solution, and 10 optical microscope micrographs are taken in five fields of view at 500 magnifications. The area of Cr-based carbides in each obtained micrograph is measured by image interpretation, and Cr-based carbides with an equivalent circular diameter of 2.0 μm or more are identified. The total area ratio of the identified Cr-based carbides with an equivalent circular diameter of 2.0 μm or more is then calculated and the calculated value is used as the volume fraction of Cr-based carbides with a grain size of 2.0 μm or more.

Here, in the image interpretation, the image interpretation device is used for the digital data of the micrograph to automatically detect the grain boundaries of the matrix phase (ferrite or martensite phase) and the boundaries of precipitates by contrast difference (grain boundaries and boundaries present a linear black contrast, while crystal grains present a relatively bright contrast). The region enclosed by the matrix phase and the boundaries of precipitates is then considered to be a precipitate, and the area of the region of each precipitate is automatically measured. Then, for precipitates identified as Cr-based carbides by the method described below, only those with an area of 3.14 μm² or more (i.e., those with an equivalent circular diameter of 2.0 μm or more) are identified. The total area of the identified precipitates is then calculated.

Then, (total area of precipitates (Cr-based carbides) with equivalent circular diameter of 2.0 μm or more)+(total area of micrograph)×100 [%] is obtained, and the obtained value is used as the volume fraction of Cr-based carbides with a grain size of 2.0 μm or more.

The precipitates in the micrograph are identified as Cr-based carbides as follows.

In detail, in the same field of view where the micrograph was taken, Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy (SEM-EDS) is used to perform point analysis to determine the main components of observed precipitates.

Specifically, when the total content of Cr and Fe in the precipitates is 60 mass % or more and the ratio of the Cr content in the precipitates to the total Fe and Cr content in the precipitates ([Cr content (mass %)]/([Fe content (mass %)]+[Cr content (mass %)]) is 0.4 or more, the precipitates are identified as Cr-based carbides.

In addition, the volume fraction of each of the ferrite and martensite phases is determined as follows.

In detail, in the micrograph, the martensite phase is distinguished from the ferrite phase based on microstructure shape and etching strength (note that the martensite phase is etched deeper than the ferrite phase. Therefore, the martensite phase has a darker contrast than the ferrite phase). The volume fractions of the ferrite and martensite phases are then calculated for each field of view by image processing. Then, the arithmetic means of the volume fractions of the ferrite and martensite phases obtained for each field of view are calculated, and the values are used as the volume fractions of the ferrite and martensite phases.

The thickness of the stainless steel sheet according to one of the disclosed embodiments is not particularly limited, but from the viewpoint of application to kitchen knives, razors, medical blades, etc., a thickness of 0.1 to 5.0 mm is suitable. The thickness of the stainless steel sheet according to one of the disclosed embodiments is more preferably 0.5 mm or more, more preferably 1.0 mm or more. The thickness of the stainless steel sheet according to one of the disclosed embodiments is more preferably 4.0 mm or less, and further preferably 2.5 mm or less.

The following describes a method of manufacturing a stainless steel sheet according to one of the disclosed embodiments.

In detail, molten steel is prepared by steelmaking in a melting furnace such as a converter or electric furnace. The molten steel is then subjected to secondary refining by ladle refining or vacuum refining to be adjusted to have the chemical composition as described above. The molten steel is then made into a steel material (steel slab) by continuous casting or ingot-casting and blooming.

—First Step (Steel Slab Heating)

As the first step, the steel slab is held at a temperature of 1200° C. or higher and 1350° C. or lower for at least 30 minutes.

Holding Steel Slab at Temperature of 1200° C. or Higher and 1350° C. or Lower for at Least 30 Minutes

In heating of the steel slab prior to hot rolling, it is necessary to dissolve the coarse Cr-based carbides formed along the casting direction near the boundaries between columnar crystals and equiaxial crystals in the steel slab section during casting to the austenite phase as much as possible.

Here, when the holding temperature of the steel slab (hereinafter referred to as “slab heating temperature”) is lower than 1200° C., the dissolution of Cr-based carbides into the austenite phase is not sufficiently promoted. Therefore, the formation of coarse Cr-based carbides is not sufficiently suppressed, and good surface quality cannot be obtained. On the other hand, when the slab heating temperature exceeds 1350° C., the metallic structure of the steel slab becomes a two-phase structure of austenite phase and delta ferrite phase or a single-phase structure of delta ferrite, and thus the dissolution of Cr-based carbides into the austenite phase is not sufficiently promoted. Therefore, the formation of coarse Cr-based carbides is not sufficiently suppressed, and good surface quality cannot be obtained.

Therefore, the slab heating temperature is in the range of 1200° C. to 1350° C. The slab heating temperature is preferably 1300° C. or lower and more preferably 1250° C. or lower.

Further, when the holding time at a temperature of 1200° C. or higher and 1350° C. or lower is less than 30 minutes, the dissolution of Cr-based carbides into the austenite phase is still insufficient. Therefore, the formation of coarse Cr-based carbides is not sufficiently suppressed, and good surface quality cannot be obtained.

Therefore, the holding time at a temperature of 1200° C. or higher and 1350° C. or lower is set to at least 30 minutes or more.

If the holding time exceeds 24 hours, the oxide scale formed during heating of the steel slab becomes thicker and surface defects are more likely to occur. It also reduces productivity. Therefore, the holding time is preferably 24 hours or less. The holding time is more preferably 12 hours or less, and further preferably 3 hours or less.

—Second Step: Hot Rolling

Then, as a second step, the steel slab is hot rolled to obtain a hot-rolled steel sheet and the hot-rolled steel sheet is coiled.

In the hot rolling, it is important to perform at least three rolling passes with a finish temperature of 1050° C. or higher and a rolling reduction of 20% or more and set a coiling temperature of the hot-rolled steel sheet to 600° C. or higher.

In hot rolling, the number of rolling passes with a finish temperature of 1050° C. or higher and a rolling reduction of 20% or more: at least three

In the hot rolling, the dissolution of Cr-based carbides into the austenite phase is further promoted to eliminate coarse Cr-based carbides remaining after steel slab heating. Further, the crystal grains of the austenite phase are refined by promoting dynamic and/or static recrystallization of the austenite phase. This increases the precipitation site of Cr-based carbides that precipitate from the grain boundaries of the austenite phase during the subsequent coiling of the hot-rolled steel sheet to refine re-precipitating Cr-based carbides.

In particular, rolling at a temperature of 1050° C. or higher effectively promotes dynamic and/or static recrystallization of the austenite phase. Further, by setting the rolling reduction to 20% or more for each rolling pass, rolling strain is effectively applied to the center portion in steel slab thickness direction. This more effectively eliminates the coarse Cr-based carbides that have formed along the casting direction near the boundaries between columnar crystals and equiaxial crystals in the steel slab.

Therefore, in the hot rolling, at least three rolling passes with a finish temperature of 1050° C. or higher and rolling reduction of 20% or more (hereinafter referred to as “rolling passes that satisfy the predetermined conditions”) need to be performed.

No upper limit is placed on the number of rolling passes that satisfy the predetermined conditions, but excessively increasing the number of rolling passes requires a large amount of heat input to maintain the rolling temperature, resulting in an increase in manufacturing costs. Therefore, the number of rolling passes that satisfy the predetermined conditions is preferably 10 or less.

No upper limit is placed on the rolling reduction for each rolling pass in the hot rolling, but when the rolling reduction for each rolling pass is excessively large, the rolling load increases and rolling becomes difficult. Therefore, the rolling reduction for each rolling pass is preferably 60% or less.

Herein, the rolling reduction for each rolling pass is calculated as ([thickness (mm) of rolled material at start of rolling pass]−[thickness (mm) of rolled material at end of rolling pass])/[thickness (mm) of rolled material at start of rolling pass]×100.

The number of rolling passes (total number of passes) in the hot rolling is preferably 8 to 20. In addition, the hot rolling generally consists of rough rolling and finish rolling. In this case, the number of rolling passes for rough rolling is preferably 3 to 10, and the number of rolling passes for finish rolling is preferably 5 to 10. The rolling finish temperature is preferably 900° C. or higher and 1100° C. or lower. Furthermore, the total rolling reduction in the hot rolling is preferably 85.0% or more and 99.8% or less.

Coiling Temperature: 600° C. or Higher

After the finish rolling of hot rolling, the hot-rolled steel sheet is coiled. At that time, the austenite phase is transformed to ferrite phase to make the metallic structure of the hot-rolled steel sheet into a microstructure mainly having ferrite phase. When the coiling temperature is lower than 600° C., the austenite phase transforms to martensite phase, resulting in hardening of the steel sheet. In addition, the flatness of the steel sheet may deteriorate, making it difficult to perform subsequent steps. Furthermore, quench crack may occur in the steel sheet.

Therefore, the coiling temperature is 600° C. or higher. The coiling temperature is preferably 650° C. or higher, more preferably 700° C. or higher, and further preferably 750° C. or higher. No upper limit is placed on the coiling temperature, but the coiling temperature is preferably 850° C. or lower. When the coiling temperature exceeds 850° C., the coiling temperature is in the dual-phase temperature range of austenite phase and ferrite phase. Therefore, the stability of the austenite phase is higher, resulting in a delay in the transformation from austenite phase to ferrite phase. This may cause the austenite phase to transform to hard martensite phase after air cooling (of the coiled steel sheet) and before hot-rolled sheet annealing. This is undesirable because it may result in significant hardening and shape defects in the hot-rolled steel sheet.

Third Step: Hot-Rolled Sheet Annealing

Then, as the third step, the hot-rolled steel sheet obtained as described above is subjected to hot-rolled sheet annealing to make a hot-rolled and annealed steel sheet.

In the hot-rolled sheet annealing, the holding temperature is 750° C. or higher and 900° C. or lower and the holding time is 10 minutes or more.

Holding temperature of hot-rolled sheet annealing: 750° C. or higher and 900° C. or lower Hot-rolled sheet annealing is performed to suppress cracking (hereinafter also referred to as “work cracking”) during working to a predetermined shape, such as an edged tool. Then, in the hot-rolled sheet annealing, recrystallization changes the rolled microstructure (metallic structure consisting of strained crystal grains) formed by the hot rolling so as to have crystal grains of ferrite phase that contain little strain.

However, when the holding temperature in the hot-rolled sheet annealing (hereinafter referred to as hot-rolled sheet annealing temperature) is lower than 750° C., the rolled microstructure formed during the hot rolling will remain. This reduces the ductility of the hot-rolled and annealed steel sheet and makes it susceptible to work cracking. When the hot-rolled sheet annealing temperature exceeds 900° C., the crystal grains coarsen and toughness decreases. Thus, work cracking easily occurs.

The hot-rolled sheet annealing temperature is therefore in a range of 750° C. to 900° C. The hot-rolled sheet annealing temperature is preferably 800° C. or higher. The hot-rolled sheet annealing temperature is preferably 875° C. or lower, and more preferably 850° C. or lower.

The hot-rolled sheet annealing temperature may be constant during the holding or may not be constant during the holding as long as it is within the above temperature range. The same applies to the cold-rolled sheet annealing temperature, quenching temperature, and tempering temperature described below.

Holding Time for Hot-Rolled Sheet Annealing: 10 Minutes or More

When the holding time of hot-rolled sheet annealing is less than 10 minutes, the material property in the steel sheet cannot be made sufficiently uniform. Therefore, the holding time for hot-rolled sheet annealing should be 10 minutes or more. The holding time for hot-rolled sheet annealing is preferably 3 hours or more, and more preferably 6 hours or more. When the holding time for hot-rolled sheet annealing exceeds 96 hours, the oxide scale may become thicker, and the subsequent descaling treatment may be difficult. Therefore, the holding time for hot-rolled sheet annealing is preferably 96 hours or less. The holding time for hot-rolled sheet annealing is preferably 24 hours or less, and more preferably 12 hours or less.

After the hot-rolled sheet annealing, cold rolling may optionally be performed as the fourth step, and further, cold-rolled sheet annealing may be performed as the fifth step.

Fourth Step: Cold Rolling

In the fourth step, the hot-rolled and annealed steel sheet obtained after hot-rolled sheet annealing is subjected to cold rolling to obtain a cold-rolled steel sheet.

The cold rolling method is not particularly limited and for example, tandem mills or cluster mills can be used. The rolling reduction in cold rolling is not particularly limited, but from the viewpoint of formability after cold-rolled sheet annealing and shape adjustment of steel sheets, the rolling reduction in cold rolling is preferably 50% or more. From the viewpoint of avoiding excessive rolling load, the rolling reduction in cold rolling should be 95% or less.

Fifth Step: Cold-Rolled Sheet Annealing

In the fifth step (cold-rolled sheet annealing), the cold-rolled steel sheet obtained after cold rolling is subjected to cold-rolled sheet annealing with a holding temperature: of 700° C. or higher and 850° C. or lower and a holding time of 5 seconds or more to make a cold-rolled and annealed steel sheet.

The main purpose of cold-rolled sheet annealing is to remove the rolled microstructure formed by cold rolling through recrystallization.

Here, when the holding temperature of cold-rolled sheet annealing (hereinafter referred to as “cold-rolled sheet annealing temperature”) is lower than 700° C., the rolled microstructure formed by cold rolling remains and the workability of the cold-rolled and annealed steel sheet obtained after cold-rolled sheet annealing decreases. On the other hand, when the holding temperature in cold-rolled sheet annealing exceeds 850° C., an austenite phase is formed, and during cooling after holding, the austenite phase transforms to a martensite phase. This leads to hardening and reduced ductility of the cold-rolled and annealed steel sheet obtained after cold-rolled sheet annealing, resulting in work cracking.

Therefore, when performing cold-rolled sheet annealing, the cold-rolled sheet annealing temperature should be in the range of 700° C. to 850° C. The cold-rolled sheet annealing temperature is preferably 720° C. or higher. The cold-rolled sheet annealing temperature is preferably 830° C. or lower.

When the holding time of cold-rolled sheet annealing is less than 5 seconds, the rolled microstructure formed by cold rolling remains and the workability of the cold-rolled and annealed steel sheet obtained after cold-rolled sheet annealing decreases. Therefore, when cold-rolled sheet annealing is performed, the holding time of cold-rolled sheet annealing should be 5 seconds or more. The holding time of cold-rolled sheet annealing is preferably 15 seconds or more.

On the other hand, when the holding time of cold-rolled sheet annealing exceeds 24 hours, crystal grains may coarsen, resulting in work cracking. Therefore, the holding time of cold-rolled sheet annealing is preferably 24 hours or less. The holding time of cold-rolled sheet annealing is more preferably 15 minutes or less.

Sixth Step: Quenching Treatment

The hot-rolled and annealed steel sheet, cold-rolled steel sheet or cold-rolled and annealed steel sheet obtained as described above is worked into a predetermined shape, for example, and then, as the sixth step, the quenching treatment with a holding temperature of 950° C. or higher and 1200° C. or lower and a holding time of 5 seconds or more and 30 minutes or less, an average cooling rate after holding: of 1° C./s or more is performed as the sixth step to make a quenched steel sheet.

When the holding temperature of the quenching treatment (hereinafter also referred to as “quenching temperature”) is lower than 950° C., the austenite phase is not sufficiently formed during heating and holding in the quenching treatment and sufficient quenching is not achieved. When the quenching temperature exceeds 1200° C., delta-ferrite phase may be formed in the metallic structure during heating and holding in the quenching treatment, which may result in insufficient quenching. In addition, crystal grains may significantly coarsen, resulting in quench cracking and work cracking during cooling.

Therefore, the quenching temperature is in the range of 950° C. to 1200° C. The quenching temperature is preferably 1000° C. or higher. The quenching temperature is preferably 1150° C. or lower.

When the holding time of the quenching treatment is less than 5 seconds, the austenite phase is not sufficiently formed during heating and holding, and sufficient quenching is not achieved. On the other hand, when the holding time in the quenching treatment exceeds 30 minutes, crystal grains may coarsen, resulting in work cracking.

Therefore, the holding time of the quenching treatment is in the range of 5 seconds to 30 minutes. The holding time in the quenching treatment is preferably 15 seconds or more. The holding time in the quenching treatment is preferably 300 seconds or less, and more preferably 120 seconds or less.

In addition, cooling is performed after holding in the quenching treatment. When the average cooling rate during the cooling, specifically, the average cooling rate within a temperature range of the quenching temperature to 400° C. is less than 1° C./s, the austenite phase formed during heating does not transform to the martensite phase but to the ferrite phase, and therefore, sufficient quenching is not achieved.

Therefore, the average cooling rate after holding in the quenching treatment is 1° C./s or more. The average cooling rate after holding in the quenching treatment is preferably 5° C./s or more, and more preferably 10° C./s or more. No upper limit is placed on the average cooling rate after holding in the quenching treatment, but excessive rapid cooling may result in deterioration of the steel sheet shape and quench cracking. Therefore, the average cooling rate after holding in the quenching treatment is preferably 1000° C./s or less.

The cooling method is not particularly limited, and various methods such as air cooling, gas injection cooling, mist water cooling, roll-chilling, water immersion, and cooling in tool can be used.

Seventh Step: Tempering Treatment

Then, in order to adjust hardness and durability, the above quenched steel sheet may be further subjected to tempering treatment as the seventh step with a holding temperature of 100° C. or higher and 800° C. or lower and a holding time of 5 minutes or more to obtain a tempered steel sheet.

When the holding temperature of the tempering treatment (hereinafter also referred to as tempering temperature) is lower than 100° C., the recovery of dislocation in the martensite phase is significantly slow. Therefore, it is difficult to sufficiently obtain the desired softening effect by tempering treatment. On the other hand, when the tempering temperature exceeds 800° C., the martensite phase transforms again to the austenite phase, and during cooling after holding, it transforms back to the martensite phase and becomes harder. Therefore, it is difficult to sufficiently obtain the desired softening effect by tempering treatment.

Therefore, the tempering temperature is in the range of 100° C. to 800° C. The tempering temperature is preferably 200° C. or higher, and more preferably 400° C. or higher. The tempering temperature is preferably 750° C. or lower, and more preferably 700° C. or lower.

When the holding time of the tempering treatment (hereinafter also referred to as tempering time) is less than 5 minutes, the recovery of dislocation in the martensite phase is insufficient. Therefore, it is difficult to sufficiently obtain the desired softening effect by tempering treatment. Thus, the tempering time is 5 minutes or more. The tempering time is preferably 10 minutes or more, and more preferably 15 minutes or more.

The hardness tends to decrease as the tempering time increases. However, when the tempering time exceeds 60 minutes, the hardness becomes almost constant. Therefore, the tempering time is preferably 60 minutes or less. The tempering time is more preferably 50 minutes or less, and further preferably 40 minutes or less.

For the conditions other than the above, conventional methods may be followed.

Pickling treatment, shot blasting, surface grinding, etc. may optionally be performed, for example, after the hot rolling step, hot-rolled sheet annealing step, cold rolling step, cold-rolled sheet annealing step, quenching step, and tempering step. Furthermore, depending on the application, temper rolling may be applied after the hot rolling step, hot-rolled sheet annealing step, cold-rolled sheet annealing step, quenching treatment step, and tempering treatment step.

The steel sheet obtained as described above can then be used to produce edged tools such as kitchen knives, scissors, and medical scalpels, cutlery such as table knives, forks and spoons, and precision tools such as tweezers.

EXAMPLES

Steels with the chemical compositions listed in Table 1 (the balance is Fe and inevitable impurities) were obtained by steelmaking via refining in a converter with a capacity of 150 tons and refining with a strongly stirred vacuum oxygen decarburization (SS-VOD) process, and then subjected to continuous casting to obtain steel slabs with a width of 1000 mm and a thickness of 200 mm.

The steel slabs were held under the conditions listed in Table 2 and subjected to hot rolling and hot-rolled sheet annealing under the conditions listed in Tables 2 and 3 to produce hot-rolled and annealed steel sheets. The (total) number of passes for hot rolling was 14. The finish temperatures of the first through fifth passes in hot rolling are higher than the finish temperature of the sixth pass, so they are omitted in Table 2. In addition, the finish temperatures of passes after the ninth pass in hot rolling are also omitted in Table 2.

Then, some of the hot-rolled and annealed steel sheets were further subjected to cold rolling and/or cold-rolled sheet annealing under the conditions listed in Table 3 to obtain cold-rolled steel sheets and/or cold-rolled and annealed steel sheets.

The hot-rolled and annealed steel sheets, cold-rolled steel sheets, and cold-rolled and annealed steel sheets thus obtained were observed for metallic structure by the method described above to identify their metallic structures. The results are listed in Table 4. However, for No. 35, cracking occurred during coiling of the hot-rolled steel sheet, so the identification of metallic structure and further evaluation were not performed.

The hot-rolled and annealed steel sheets, cold-rolled steel sheets, and cold-rolled and annealed steel sheets obtained as described above were punched into a shape of 300 mm in rolling direction ×50 mm in width direction. Then, the worked steel sheets were subjected to quenching treatment with air cooling under the following conditions: quenching temperature: 1050° C., holding time: 15 minutes, and average cooling rate in the temperature range of a quenching temperature after holding to 400° C.: 5° C./s.

Note that Nos. 1A, 3A-1, and 3A-2 are steel sheets (tempered steel sheets) obtained by further subjecting the steel sheets of Nos. 1 and 3 after quenching treatment to tempering treatment under the conditions listed in Table 3.

The quenched steel sheets and tempered steel sheets thus obtained were observed for metallic structure by the method described above to identify their metallic structures. The results are listed in Table 4.

In addition, hardness and surface quality were evaluated according to the following procedures.

The hardness was evaluated using quenched steel sheets. However, for No. 1A and 3A-1 and 3A-2 where tempering treatment was performed, the hardness was evaluated on the steel sheets after tempering treatment.

The evaluation of surface quality was performed on the finally obtained steel sheets, i.e., the quenched steel sheets for Nos. 1 to 37, and the tempered steel sheets for Nos. 1A, 3A-1, and 3A-2.

<Hardness Evaluation>

The Rockwell hardness test in accordance with JIS Z 2245 (2016) was performed at five arbitrary points on the rolled surface of each steel sheet obtained as described above. The average value of Rockwell hardness at the five points was obtained. The rolled surface of the steel sheet was surface polished with #400 water-resistant emery polishing paper before the test. The hardness was then evaluated according to the following criteria. The evaluation results are listed in Table 4.

-   -   When tempering treatment was not performed         -   Passed: Average Rockwell hardness of HRC55 or more         -   Failed: Average Rockwell hardness of less than HRC55     -   When tempering treatment was performed         -   Passed: Average Rockwell hardness before tempering treatment             of HRC55 or more and average Rockwell hardness after             tempering treatment of HRC40 or more         -   Failed: Average Rockwell hardness before tempering treatment             of less than HRC55 and average Rockwell hardness after             tempering treatment of less than HRC40

<Evaluation of Surface Quality>

From each steel sheet obtained as described above, ten test pieces of 100 mm in rolling direction ×50 mm in width direction were collected. Then, as illustrated in FIG. 3 , in each test piece, one of the end surfaces parallel to the rolling direction and width direction was applied with cutting work at an angle of 3.5° with respect to the width direction. The cut surface was then subjected to wet cross polishing (polishing where polishing of the following number is done in the direction orthogonal to the polishing direction of polishing of the previous number) using water-resistant emery papers in the order of #400, #600, #800, #1200, and #2000 to thereby obtain a polished and edged surface.

Then, the polished and edged surface was visually observed and evaluated for surface quality according to the following criteria. The evaluation results are listed in Table 4.

Passed: No stripe pattern of 2.0 mm or more in length is observed on the polished and edged surface in all ten test pieces.

Failed: Any stripe pattern of 2.0 mm or more in length is observed on the polished and edged surface in at least one of the ten test pieces.

TABLE 1 Steel sample Chemical Composition (mass %) ID C Si Mn P S Cr Ni N Others (1) Others (2) Remarks A1 0.46 0.39 0.57 0.03 0.008 15.4 0.21 0.044 — — Conforming steel A2 0.48 0.41 0.61 0.02 0.001 14.5 0.48 0.035 Mo: 0.53 — Conforming steel A3 0.50 0.42 0.58 0.02 0.002 14.4 0.22 0.031 Cu: 0.21, Co: 0.07 — Conforming steel A4 0.45 0.37 0.56 0.01 0.003 14.4 0.52 0.087 — B: 0.0011, Mg: 0.0005, REM: 0.02 Conforming steel A5 0.52 0.44 0.63 0.01 0.002 14.1 0.46 0.193 — Al: 0.05, Ti: 0.07, Nb: 0.09, V: 0.14 Conforming steel A6 0.46 0.38 0.61 0.02 0.004 14.3 0.44 0.042 — Zr: 0.06, B: 0.0003, Ca: 0.0011 Conforming steel A7 0.49 0.42 0.66 0.01 0.003 14.3 0.50 0.038 Mo: 0.44 V: 0.21, B: 0.0003 Conforming steel A8 0.47 0.40 0.61 0.01 0.001 14.2 0.49 0.029 — — Conforming steel B1 0.34 0.46 0.37 0.02 0.001 13.3 0.26 0.027 — — Comparative steel A9 0.49 0.07 0.60 0.03 0.002 14.3 0.47 0.036 — — Conforming steel A10 0.48 0.96 0.62 0.03 0.003 14.2 0.46 0.035 — — Conforming steel A11 0.48 0.40 0.06 0.02 0.002 14.6 0.48 0.041 — — Conforming steel A12 0.50 0.37 0.98 0.03 0.003 14.4 0.45 0.039 — — Conforming steel A13 0.51 0.39 0.59 0.03 0.004 14.5 0.12 0.042 — — Conforming steel A14 0.49 0.38 0.60 0.03 0.004 14.3 0.97 0.037 — — Conforming steel

TABLE 2 Hot Rolling Rolling Rolling Rolling Rolling Rolling Rolling Rolling Rolling Slab Heating reduction reduction reduction reduction reduction reduction reduction reduction Slab Start of of of of of of of of Steel heating Holding Start sheet first second third fourth fifth sixth seventh eigth sample temperature time temperature thickness pass pass pass pass pass pass pass pass No. ID [° C.] [h] [° C.] [mm] [%] [%] [%] [%] [%] [%] [%] [%] Remarks 1, 1A A1 1220 1 1213 200 18 28 27 26 18 18 18 14 Ex.  2 A1 1217 1 1209 200 19 27 28 27 18 17 17 14 Ex. 3, 3A-1, A1 1215 1 1207 200 18 27 29 28 18 17 16 16 Ex. 3A-2  4 A2 1221 1 1212 200 17 28 29 26 18 17 16 13 Ex.  5 A3 1214 1 1208 200 18 27 27 27 18 17 18 16 Ex.  6 A3 1344 1 1306 200 18 28 29 27 17 16 16 13 Ex.  7 A3 1207 1 1199 200 18 28 30 23 18 18 18 13 Ex.  8 A4 1219 1 1212 200 18 29 27 27 17 17 17 13 Ex.  9 A4 1218 1 1135 200 18 27 27 29 17 18 18 15 Ex. 10 A5 1219 1 1212 200 17 30 28 25 18 18 17 16 Ex. 11 A6 1221 1 1217 200 18 26 26 29 18 18 18 15 Ex. 12 A7 1224 1 1217 200 17 29 29 27 17 18 16 15 Ex. 13 A7 1220 1 1214 200 18 29 25 29 17 18 17 13 Ex. 14 A7 1223 1 1216 200 16 30 23 29 18 18 18 16 Ex. 15 A8 1226 1 1218 200 18 28 27 28 18 18 16 13 Ex. 16 A8 1217 1 1210 200 17 28 26 29 18 18 17 16 Ex. 17, 18, A8 1215 1 1209 200 18 28 29 27 17 15 16 16 Ex. 19, 20 21 A1 1217 1 1207 200 19 25 28 28 17 17 16 16 Ex. 22 A1 1218 1 1210 200 18 27 27 28 18 17 16 15 Ex. 23 A1 1218 1 1211 200 18 28 28 29 18 16 16 14 Ex. 24 A8 1221 1 1211 200 18 29 27 27 18 16 16 17 Ex. 25 A8 1219 1 1210 200 18 28 28 28 18 16 17 15 Ex. 26 A8 1215 1 1209 200 17 28 28 29 17 16 17 16 Ex. 27 A8 1218 1 1212 200 19 27 28 28 18 16 17 15 Ex. 28 A8 1217 1 1212 200 18 26 29 28 17 17 18 16 Ex. 29 A8 1219 1 1208 200 18 28 28 28 18 16 17 15 Ex. 30 A1 1217 1 1208 200 18 18 29 27 18 17 18 14 Com Ex. 31 A3 1359 1 1342 200 18 28 28 25 18 18 18 16 Com Ex. 32 A3 1177 1 1164 200 17 27 27 29 17 18 18 14 Com Ex. 33 A4 1208 1 1199 200 18 19 18 18 27 26 26 13 Com Ex. 34 A4 1223 1 1218 200 18 17 17 18 32 18 18 16 Com Ex. 35 A7 1215 1 1209 200 17 26 29 27 18 18 18 13 Com Ex. 36 B1 1218 1 1211 200 18 18 18 17 18 19 18 13 Com Ex. 37 B1 1222 1 1213 200 17 27 29 28 18 17 17 15 Com Ex. 38 A9 1217 1 1206 200 17 28 26 29 18 18 18 16 Ex. 39  A10 1215 1 1208 200 18 27 28 27 18 18 18 16 Ex. 40  A11 1215 1 1208 200 18 27 27 28 17 18 18 17 Ex. 41, 42  A12 1216 1 1204 200 17 27 29 28 18 18 18 18 Ex. 43  A13 1218 1 1207 200 18 29 27 26 18 17 17 16 Ex. 44  A14 1215 1 1204 200 17 29 27 28 18 18 16 16 Ex. 45  A12 1214 1 1205 200 17 28 28 29 19 18 17 16 Ex. 46  A12 1215 1 1206 200 18 28 27 29 18 18 19 16 Ex. Hot Rolling Sheet Sheet Sheet Sheet Sheet Number of Sheet Sheet thickness thickness thickness Sheet Sheet thickness thickness Finish Finish Finish rolling thickness thickness at finish at finish at finish thickness thickness at finish of at finish of temperature temperature temperature passes at finish Steel at finish of of of of at finish of at finish of seventh eighth at sixth at seventh at eighth satisfying of Coiling sample first pass second pass third pass fourth pass fifth pass sixth pass pass pass pass pass pass predetermined rolling temperature No. ID [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [° C.] [° C.] [° C.] conditions [mm] [° C.] Remarks 1, 1A A1 164.0 118.1 86.2 63.8 52.3 42.9 35.2 30.2 1081 1060 1042 3 4.1 766 Ex.  2 A1 162.0 118.3 85.1 62.2 51.0 42.3 35.1 30.2 1084 1062 1042 3 4.0 768 Ex. 3, 3A-1, A1 164.0 119.7 85.0 61.2 50.2 41.7 35.0 29.4 1082 1063 1044 3 3.9 773 Ex. 3A-2  4 A2 166.0 119.5 84.9 62.8 51.5 42.7 35.9 31.2 1084 1064 1046 3 3.8 771 Ex.  5 A3 164.0 119.7 87.4 63.8 52.3 43.4 35.6 29.9 1086 1066 1046 3 3.9 776 Ex.  6 A3 164.0 118.1 83.8 61.2 50.8 42.7 35.8 31.2 1082 1061 1044 3 3.7 789 Ex.  7 A3 164.0 118.1 82.7 63.6 52.2 42.8 35.1 30.5 1082 1061 1044 3 4.2 754 Ex.  8 A4 164.0 116.4 85.0 62.1 51.5 42.7 35.5 30.9 1087 1067 1046 3 4.1 780 Ex.  9 A4 164.0 119.7 87.4 62.1 51.5 42.2 34.6 29.4 1081 1063 1045 3 4.0 762 Ex. 10 A5 166.0 116.2 83.7 62.7 51.5 42.2 35.0 29.4 1082 1060 1039 3 4.1 780 Ex. 11 A6 164.0 121.4 89.8 63.8 52.3 42.9 35.2 29.9 1086 1064 1046 3 4.0 771 Ex. 12 A7 166.0 117.9 83.7 61.1 50.7 41.6 34.9 29.7 1078 1060 1040 3 3.9 784 Ex. 13 A7 164.0 116.4 87.3 62.0 51.5 42.2 35.0 30.5 1084 1062 1042 3 4.4 770 Ex. 14 A7 168.0 117.6 90.6 64.3 52.7 43.2 35.4 29.8 1082 1063 1046 3 4.4 770 Ex. 15 A8 164.0 118.1 86.2 62.1 50.9 41.7 35.1 30.5 1078 1058 1041 3 4.4 771 Ex. 16 A8 166.0 119.5 88.4 62.8 51.5 42.2 35.0 29.4 1084 1065 1044 3 4.2 769 Ex. 17, 18, A8 164.0 118.1 83.8 61.2 50.8 43.2 36.3 30.5 1082 1062 1044 3 4.3 777 Ex. 19, 20 21 A1 162.0 121.5 87.5 63.0 52.3 43.4 36.4 30.6 1081 1059 1047 3 3.9 779 Ex. 22 A1 164.0 119.7 87.4 62.9 51.6 42.8 36.0 30.6 1084 1064 1044 3 4.0 766 Ex. 23 A1 164.0 118.1 85.0 60.4 49.5 41.6 34.9 30.0 1082 1061 1041 3 3.9 780 Ex. 24 A8 164.0 116.4 85.0 62.1 50.9 42.7 35.9 29.8 1083 1066 1042 3 3.8 773 Ex. 25 A8 164.0 118.1 85.0 61.2 50.2 42.2 35.0 29.7 1079 1061 1037 3 4.0 777 Ex. 26 A8 166.0 119.5 86.1 61.1 50.7 42.6 35.4 29.7 1084 1067 1039 3 3.9 781 Ex. 27 A8 162.0 118.3 85.1 61.3 50.3 42.2 35.0 29.8 1082 1064 1040 3 3.8 784 Ex. 28 A8 164.0 121.4 86.2 62.0 51.5 42.7 35.0 29.4 1080 1065 1042 3 3.9 779 Ex. 29 A8 164.0 118.1 85.0 61.2 50.2 42.2 35.0 29.7 1081 1068 1041 3 4.0 776 Ex. 30 A1 164.0 134.5 95.5 69.7 57.2 47.4 38.9 33.5 1081 1061 1043 2 4.4 778 Com Ex. 31 A3 164.0 118.1 85.0 63.8 52.3 42.9 35.2 29.5 1187 1169 1149 3 3.8 769 Com Ex. 32 A3 166.0 121.2 88.5 62.8 52.1 42.7 35.1 30.1 1085 1065 1045 3 4.1 768 Com Ex. 33 A4 164.0 132.8 108.9 89.3 65.2 48.3 35.7 31.1 1057 989 971 2 3.9 771 Com Ex. 34 A4 164.0 136.1 113.0 92.6 63.0 51.7 42.4 35.6 1082 1060 1041 1 3.9 771 Com Ex. 35 A7 166.0 122.8 87.2 63.7 52.2 42.8 35.1 30.5 1081 1060 1041 3 4.2 539 Com Ex. 36 B1 164.0 134.5 110.3 91.5 75.1 60.8 49.8 43.4 1083 1063 1042 0 4.1 766 Com Ex. 37 B1 166.0 121.2 86.0 61.9 50.8 42.2 35.0 29.7 1085 1066 1046 3 3.9 773 Com Ex. 38 A9 166.0 119.5 88.4 62.8 51.5 42.2 34.6 29.1 1087 1069 1047 3 4.0 777 Ex. 39  A10 164.0 119.7 86.2 62.9 51.6 42.3 34.7 29.1 1084 1064 1043 3 4.2 780 Ex. 40  A11 164.0 119.7 87.4 62.9 52.2 42.8 35.1 29.1 1086 1068 1047 3 4.1 774 Ex. 41, 42  A12 166.0 121.2 86.0 61.9 50.8 41.7 34.2 28.0 1084 1064 1043 3 4.0 782 Ex. 43  A13 164.0 116.4 85.0 62.9 51.6 42.8 35.5 29.8 1084 1064 1043 3 4.2 770 Ex. 44  A14 166.0 117.9 86.0 61.9 50.8 41.7 35.0 29.4 1088 1069 1049 3 4.3 776 Ex. 45  A12 166.0 119.5 86.1 61.1 49.5 40.6 33.7 28.3 1077 1059 1046 3 4.1 606 Ex. 46  A12 164.0 118.1 86.2 61.2 50.2 41.2 33.3 28.0 1079 1063 1050 3 4.0 658 Ex.

TABLE 3 Cold rolling Hot-rolled sheet annealing Sheet thickness Cold-rolled sheet annealing Quenching treatment Tempering treatment Steel Annealing Holding at finish of Annealing Holding Quenching Holding Holding Holding sample temperature time cold rolling temperature time temperature time temperature time No. ID [° C.] M [mm] [° C.] [s] [° C.] [min] [° C.] [min] Remarks 1 A1 833 8 not performed not performed 1049 15 not performed Ex. 2 A1 830 8 2.0 not performed 1051 15 not performed Ex. 3 A1 826 8 2.0 827 62 1052 15 not performed Ex. 4 A2 833 8 not performed not performed 1050 15 not performed Ex. 5 A3 846 8 not performed not performed 1048 15 not performed Ex. 6 A3 843 8 not performed not performed 1053 15 not performed Ex. 7 A3 843 8 not performed not performed 1049 15 not performed Ex. 8 A4 844 8 not performed not performed 1050 15 not performed Ex. 9 A4 845 8 not performed not performed 1051 15 not performed Ex. 10 A5 845 8 not performed not performed 1050 15 not performed Ex. 11 A6 843 8 not performed not performed 1052 15 not performed Ex. 12 A7 839 8 not performed not performed 1048 15 not performed Ex. 13 A7 842 8 2.0 not performed 1050 15 not performed Ex. 14 A7 842 8 2.0 832 74 1049 15 not performed Ex. 15 A8 828 8 not performed not performed 1047 15 not performed Ex. 16 A8 833 8 2.0 not performed 1050 15 not performed Ex. 17 A8 830 8 2.0 826 61 1051 15 not performed Ex. 18 A8 830 8 2.0 826 61 984 15 not performed Ex. 19 A8 830 8 2.0 826 61 1050 0.5 not performed Ex. 20 A8 830 8 2.0 826 61 1048 30 not performed Ex. 21 A1 804 8 not performed not performed 1047 15 not performed Ex. 22 A1 872 8 not performed not performed 1053 15 not performed Ex. 23 A1 838 8 1.5 827 61 1050 15 not performed Ex. 24 A8 831 0.25 not performed not performed 1049 15 not performed Ex. 25 A8 835 24 not performed not performed 1050 15 not performed Ex. 26 A8 833 8 2.0 734 62 1050 15 not performed Ex. 27 A8 830 8 2.0 836 7 1048 15 not performed Ex. 28 A8 832 8 2.0 827 900 1050 15 not performed Ex. 29 A8 834 8 2.0 805 28800 1050 15 not performed Ex. 30 A1 836 8 not performed not performed 1050 15 not performed Com. Ex. 31 A3 845 8 not performed not performed 1047 15 not performed Com. Ex. 32 A3 846 8 not performed not performed 1050 15 not performed Com. Ex. 33 A4 841 8 not performed not performed 1049 15 not performed Com. Ex. 34 A4 845 8 not performed not performed 1050 15 not performed Com. Ex. 35 A7 omitted due to crack in hot-rolled steel sheet Com. Ex. 36 B1 835 8 not performed not performed 1051 15 not performed Com. Ex. 37 B1 831 8 2.0 830 61 1051 15 not performed Com. Ex. 1A A1 833 8 not performed not performed 1049 15 550 15 Ex. 3A-1 A1 826 8 2.0 827 62 1052 15 550 15 Ex. 3A-2 A1 826 8 2.0 827 62 1052 15 400 15 Ex. 38 A9 832 8 not performed not performed 1051 15 not performed Ex. 39  A10 831 8 not performed not performed 1050 15 not performed Ex. 40  A11 830 8 not performed not performed 1052 15 not performed Ex. 41  A12 833 8 not performed not performed 1051 15 not performed Ex. 42  A12 760 8 not performed not performed 1051 15 not performed Ex. 43  A13 829 8 not performed not performed 1049 15 not performed Ex. 44  A14 832 8 not performed not performed 1053 15 not performed Ex. 45  A12 830 8 not performed not performed 1056 15 not performed Ex. 46  A12 829 8 not performed not performed 1055 15 not performed Ex.

TABLE 4 Steel Microstructure Before quenching treatment After quenching treatment or after tempering treatment Volume Volume fraction of fraction of Cr-based Cr-based Volume Evaluation Result carbides with Volume Volume Volume carbides with Volume fraction of Hardness grain size of fraction of fraction of fraction of grain size of fraction of Volume residual Hardness Steel 2.0 μm or martensite ferrite residual 2.0 μm or martensite fraction of micro- after sample more phase phase microstructure more phase ferrite phase structure Hardness tempering Surface Re- No. ID Subject [%] [%] [%] [%] Subject [%] [%] [%] [%] (HRC) (HRC) Evaluation quality marks 1 A1 hot-rolled and 3 0 85 12 quenched 3 83 0 14 55 — passed passed Ex. annealed steel sheet steel sheet 2 A1 cold-rolled steel 3 0 84 13 quenched 3 83 0 14 59 — passed passed Ex. sheet steel sheet 3 A1 cold-rolled and 3 0 84 13 quenched 3 84 0 13 60 — passed passed Ex. annealed steel sheet steel sheet 4 A2 hot-rolled and 3 0 86 11 quenched 3 85 0 12 58 — passed passed Ex. annealed steel sheet steel sheet 5 A3 hot-rolled and 4 0 86 10 quenched 4 84 0 12 59 — passed passed Ex. annealed steel sheet steel sheet 6 A3 hot-rolled and 1 0 84 15 quenched 1 82 0 17 60 — passed passed Ex. annealed steel sheet steel sheet 7 A3 hot-rolled and 7 0 85 8 quenched 7 83 0 10 56 — passed passed Ex. annealed steel sheet steel sheet 8 A4 hot-rolled and 4 0 85 11 quenched 4 83 0 13 58 — passed passed Ex. annealed steel sheet steel sheet 9 A4 hot-rolled and 5 0 85 10 quenched 5 84 0 11 58 — passed passed Ex. annealed steel sheet steel sheet 10 A5 hot-rolled and 5 0 84 11 quenched 5 83 0 12 59 — passed passed Ex. annealed steel sheet steel sheet 11 A6 hot-rolled and 5 0 85 10 quenched 5 83 0 12 60 — passed passed Ex. annealed steel sheet steel sheet 12 A7 hot-rolled and 4 0 86 10 quenched 4 85 0 11 60 — passed passed Ex. annealed steel sheet steel sheet 13 A7 cold-rolled steel 5 0 84 11 quenched 5 83 0 12 59 — passed passed Ex. sheet steel sheet 14 A7 cold-rolled and 4 0 86 10 quenched 4 85 0 11 61 — passed passed Ex. annealed steel sheet steel sheet 15 A8 hot-rolled and 3 0 85 12 quenched 3 83 0 14 55 — passed passed Ex. annealed steel sheet steel sheet 16 A8 cold-rolled steel 3 0 85 12 quenched 3 84 0 13 55 — passed passed Ex. sheet steel sheet 17 A8 cold-rolled and 3 0 86 11 quenched 3 84 0 13 55 — passed passed Ex. annealed steel sheet steel sheet 18 A8 cold-rolled and 3 0 86 11 quenched 3 81 0 16 58 — passed passed Ex. annealed steel sheet steel sheet 19 A8 cold-rolled and 5 0 84 11 quenched 5 86 0 9 56 — passed passed Ex. annealed steel sheet steel sheet 20 A8 cold-rolled and 6 0 86 8 quenched 6 88 0 6 58 — passed passed Ex. annealed steel sheet steel sheet 21 A1 hot-rolled and 8 0 86 6 quenched 8 87 0 5 59 — passed passed Ex. annealed steel sheet steel sheet 22 A1 hot-rolled and 2 0 84 14 quenched 2 83 0 15 59 — passed passed Ex. annealed steel sheet steel sheet 23 A1 cold-rolled and 4 0 85 11 quenched 4 87 0 9 59 — passed passed Ex. annealed steel sheet steel sheet 24 A8 hot-rolled and 6 0 85 9 quenched 6 84 0 10 57 — passed passed Ex. annealed steel sheet steel sheet 25 A8 hot-rolled and 3 0 84 13 quenched 3 82 0 15 56 — passed passed Ex. annealed steel sheet steel sheet 26 A8 cold-rolled and 2 0 84 14 quenched 2 83 0 15 58 — passed passed Ex. annealed steel sheet steel sheet 27 A8 cold-rolled and 3 0 86 11 quenched 3 88 0 9 56 — passed passed Ex. annealed steel sheet steel sheet 28 A8 cold-rolled and 3 0 85 12 quenched 3 85 0 12 58 — passed passed Ex. annealed steel sheet steel sheet 29 A8 cold-rolled and 2 0 85 13 quenched 2 87 0 11 56 — passed passed Ex. annealed steel sheet steel sheet 30 A1 hot-rolled and 12  0 85 3 quenched 12  83 0 5 56 — passed failed Com annealed steel sheet steel sheet Ex. 31 A3 hot-rolled and 12  0 84 4 quenched 12  84 0 4 59 — passed failed Com annealed steel sheet steel sheet Ex. 32 A3 hot-rolled and 11  0 85 4 quenched 11  84 0 5 57 — passed failed Com annealed steel sheet steel sheet Ex. 33 A4 hot-rolled and 11  0 84 5 quenched 11  84 0 5 58 — passed failed Com annealed steel sheet steel sheet Ex. 34 A4 hot-rolled and 11  0 85 4 quenched 11  83 0 6 59 — passed failed Com annealed steel sheet steel sheet Ex. 35 A7 omitted due to crack in hot-rolled steel sheet Com Ex. 36 B1 hot-rolled and 3 0 84 13 quenched 3 83 0 14 51 — failed passed Com annealed steel sheet steel sheet Ex. 37 B1 cold-rolled and 3 0 86 11 quenched 3 86 0 11 52 — failed passed Com annealed steel sheet steel sheet Ex. 1A A1 hot-rolled and 3 0 85 12 quenched 3 83 0 14 55 44 passed passed Ex. annealed steel sheet steel sheet 3A-1 A1 cold-rolled and 3 0 84 13 quenched 3 86 0 11 60 45 passed passed Ex. annealed steel sheet steel sheet 3A-2 A1 cold-rolled and 4 0 85 11 quenched 4 85 0 11 60 53 passed passed Ex. annealed steel sheet steel sheet 38 A9 hot-rolled and 2 0 88 10 quenched 2 89 0 9 55 — passed passed Ex. annealed steel sheet steel sheet 39 A10 hot-rolled and 4 0 84 12 quenched 4 86 0 10 56 — passed passed Ex. annealed steel sheet steel sheet 40 A11 hot-rolled and 3 0 83 14 quenched 3 86 0 11 55 — passed passed Ex. annealed steel sheet steel sheet 41 A12 hot-rolled and 2 0 82 16 quenched 2 82 0 16 57 — passed passed Ex. annealed steel sheet steel sheet 42 A12 hot-rolled and 6 0 85 9 quenched 6 83 0 11 55 — passed passed Ex. annealed steel sheet steel sheet 43 A13 hot-rolled and 4 0 86 10 quenched 4 88 0 8 55 — passed passed Ex. annealed steel sheet steel sheet 44 A14 hot-rolled and 3 0 79 18 quenched 3 85 0 12 57 — passed passed Ex. annealed steel sheet steel sheet 45 A12 hot-rolled and 4 0 86 10 quenched 4 85 0 11 58 — passed passed Ex. annealed steel sheet steel sheet 46 A12 hot-rolled and 3 0 85 12 quenched 3 84 0 13 58 — passed passed Ex. annealed steel sheet steel sheet

As listed in Table 4, Examples all had high hardness and good surface quality.

On the other hand, in Comparative Examples Nos. 30, 33 and 34, since the number of rolling passes satisfying the prescribed conditions in the hot rolling was less than 3, the total volume fraction of Cr-based carbides with a grain size of 2.0 μm or more exceeded 10%. Therefore, good surface quality could not be obtained.

In No. 31, since the slab heating temperature exceeded the appropriate range, the total volume fraction of Cr-based carbides with a grain size of 2.0 μm or more exceeded 10%. Therefore, good surface quality could not be obtained.

In No. 32, since the slab heating temperature did not reach the appropriate range, the total volume fraction of Cr-based carbides with a grain size of 2.0 μm or more exceeded 10%. Therefore, good surface quality could not be obtained.

In No. 35, cracking occurred in the hot-rolled steel sheet since the coiling temperature of hot rolling did not reach the appropriate range.

In No. 36 and 37, since the C content did not reach the appropriate range, the hardness after quenching treatment did not reach the appropriate range. In No. 36, since the C content did not reach the appropriate range, although the number of rolling passes satisfying the prescribed conditions in the hot rolling was less than 3, the total volume fraction of Cr-based carbides with a grain size of 2.0 μm or more was 10% or less.

For reference, an optical microscope micrograph of a cross section parallel to the rolling direction of Example No. 1, where good surface quality was obtained, is illustrated in FIG. 1 . In addition, an optical microscope micrograph of a cross section parallel to the rolling direction of Comparative Example No. 30, where good surface quality was not obtained, is illustrated in FIG. 2 .

INDUSTRIAL APPLICABILITY

Since the stainless steel sheet of this disclosure has high hardness and good surface quality, it can be suitably used as materials of edged tools such as kitchen knives, scissors, and medical scalpels, cutlery such as table knives, forks and spoons, and precision tools such as tweezers. 

1. A stainless steel sheet comprising a chemical composition containing, in mass %, C: 0.45% or more and 0.60% or less, Si: 0.05% or more and 1.00% or less, Mn: 0.05% or more and 1.00% or less, P: 0.05% or less, S: 0.020% or less, Cr: 13.0% or more and less than 16.0%, Ni: 0.10% or more and 1.00% or less, and N: 0.010% or more and 0.200% or less with the balance being Fe and inevitable impurities, wherein a total volume fraction of Cr-based carbides with a grain size of 2.0 μm or more is 10% or less.
 2. The stainless steel sheet according to claim 1, wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of Mo: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, and Co: 0.05% or more and 0.50% or less.
 3. The stainless steel sheet according to claim 1, wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of Al: 0.001% or more and 0.100% or less, Ti: 0.01% or more and 0.10% or less Nb: 0.01% or more and 0.10% or less, V: 0.05% more and 0.50% or less, Zr: 0.01% or more and 0.10% or less, Mg: 0.0002% or more and 0.0050% or less, B: 0.0002% or more and 0.0050% or less, Ca: 0.0003% or more and 0.0030% or less, and REM: 0.01% or more and 0.10% or less.
 4. A method of manufacturing the stainless steel according to claim 1, the method comprising: a first step in which a steel slab having the chemical composition according to claim 1 is held at 1200° C. or higher and 1350° C. or lower for at least 30 minutes, a second step in which the steel slab is hot rolled to obtain a hot-rolled steel sheet and the hot-rolled steel sheet is coiled, and a third step in which the hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled and annealed steel sheet wherein in hot rolling of the second step, at least three rolling passes with a finish temperature of 1050° C. or higher and a rolling reduction of 20% or more are performed, and a coiling temperature of the hot-rolled steel sheet is 600° C. or higher, and in the hot-rolled sheet annealing of the third step, a holding temperature is 750° C. or higher and 900° C. or lower and a holding time is 10 minutes or more. 5-10. (canceled)
 11. The stainless steel sheet according to claim 2, wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of Al: 0.001% or more and 0.100% or less, Ti: 0.01% or more and 0.10% or less Nb: 0.01% or more and 0.10% or less, V: 0.05% more and 0.50% or less, Zr: 0.01% or more and 0.10% or less, Mg: 0.0002% or more and 0.0050% or less, B: 0.0002% or more and 0.0050% or less, Ca: 0.0003% or more and 0.0030% or less, and REM: 0.01% or more and 0.10% or less.
 12. A method of manufacturing the stainless steel according to claim 2, the method comprising: a first step in which a steel slab having the chemical composition according to claim 2 is held at 1200° C. or higher and 1350° C. or lower for at least 30 minutes, a second step in which the steel slab is hot rolled to obtain a hot-rolled steel sheet and the hot-rolled steel sheet is coiled, and a third step in which the hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled and annealed steel sheet wherein in hot rolling of the second step, at least three rolling passes with a finish temperature of 1050° C. or higher and a rolling reduction of 20% or more are performed, and a coiling temperature of the hot-rolled steel sheet is 600° C. or higher, and in the hot-rolled sheet annealing of the third step, a holding temperature is 750° C. or higher and 900° C. or lower and a holding time is 10 minutes or more.
 13. A method of manufacturing the stainless steel according to claim 3, the method comprising: a first step in which a steel slab having the chemical composition according to claim 3 is held at 1200° C. or higher and 1350° C. or lower for at least 30 minutes, a second step in which the steel slab is hot rolled to obtain a hot-rolled steel sheet and the hot-rolled steel sheet is coiled, and a third step in which the hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled and annealed steel sheet wherein in hot rolling of the second step, at least three rolling passes with a finish temperature of 1050° C. or higher and a rolling reduction of 20% or more are performed, and a coiling temperature of the hot-rolled steel sheet is 600° C. or higher, and in the hot-rolled sheet annealing of the third step, a holding temperature is 750° C. or higher and 900° C. or lower and a holding time is 10 minutes or more.
 14. A method of manufacturing the stainless steel according to claim 11, the method comprising: a first step in which a steel slab having the chemical composition according to claim 11 is held at 1200° C. or higher and 1350° C. or lower for at least 30 minutes, a second step in which the steel slab is hot rolled to obtain a hot-rolled steel sheet and the hot-rolled steel sheet is coiled, and a third step in which the hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled and annealed steel sheet wherein in hot rolling of the second step, at least three rolling passes with a finish temperature of 1050° C. or higher and a rolling reduction of 20% or more are performed, and a coiling temperature of the hot-rolled steel sheet is 600° C. or higher, and in the hot-rolled sheet annealing of the third step, a holding temperature is 750° C. or higher and 900° C. or lower and a holding time is 10 minutes or more.
 15. The method of manufacturing the stainless steel sheet according to claim 14, comprising the fourth step in which the hot-rolled and annealed steel sheet is cold rolled to obtain a cold-rolled steel sheet.
 16. The method of manufacturing the stainless steel sheet according to claim 15, comprising the fifth step in which the cold-rolled steel sheet is subjected to cold-rolled sheet annealing to obtain a cold-rolled and annealed steel sheet wherein in the cold-rolled sheet annealing, a holding temperature is 700° C. or higher and 850° C. or lower and a holding time is 5 seconds or more.
 17. The method of manufacturing the stainless steel sheet according to claim 14, comprising the sixth step in which the hot-rolled and annealed steel sheet is subjected to quenching treatment wherein in the quenching treatment, a holding temperature is 950° C. or higher and 1200° C. or lower, a holding time is 5 seconds or more and 30 minutes or less, and an average cooling rate after holding is 1° C./s or more.
 18. The method of manufacturing the stainless steel sheet according to claim 15, comprising the sixth step in which the cold-rolled steel sheet is subjected to quenching treatment wherein in the quenching treatment, a holding temperature is 950° C. or higher and 1200° C. or lower, a holding time is 5 seconds or more and 30 minutes or less, and an average cooling rate after holding is 1° C./s or more.
 19. The method of manufacturing the stainless steel sheet according to claim 16, comprising the sixth step in which the cold-rolled and annealed steel sheet is subjected to quenching treatment wherein in the quenching treatment, a holding temperature is 950° C. or higher and 1200° C. or lower, a holding time is 5 seconds or more and 30 minutes or less, and an average cooling rate after holding is 1° C./s or more.
 20. The method of manufacturing the stainless steel sheet according to claim 17, comprising the seventh step in which the steel sheet subjected to quenching treatment is subjected to a tempering treatment wherein in the tempering treatment, a holding temperature is 100° C. or higher and 800° C. or lower and a holding time is 5 minutes or more.
 21. The method of manufacturing the stainless steel sheet according to claim 18, comprising the seventh step in which the steel sheet subjected to quenching treatment is subjected to a tempering treatment wherein in the tempering treatment, a holding temperature is 100° C. or higher and 800° C. or lower and a holding time is 5 minutes or more.
 22. The method of manufacturing the stainless steel sheet according to claim 19, comprising the seventh step in which the steel sheet subjected to quenching treatment is subjected to a tempering treatment wherein in the tempering treatment, a holding temperature is 100° C. or higher and 800° C. or lower and a holding time is 5 minutes or more.
 23. An edged tool made of the stainless steel sheet according to claim
 11. 24. Cutlery made of the stainless steel sheet according to claim
 11. 